This invention relates to digital signal processing and, more particularly, to analog-to-digital conversion in digital message transmission systems of a coarsely quantized digital signal into a high resolution digital signal including code groups of several digit positions.
A number of prior art encoders have been developed which utilize high speed sampling and encoding at many times the Nyquist rate to make up for a lack of precision or accuracy in their operation. Such a tradeoff significantly reduces the amount of hardware otherwise required by a straightforward analog-to-digital conversion of each sampled analog signal into a high resolution code word or group of several digits. In one of these arrangements, a number of digits from a high speed delta modulator are combined by digital multipliers to produce a pulse code modulated (PCM) signal. A disadvantage of this arrangement is that the delta modulator must operate at extremely high speeds to provide a high resolution PCM signal. If a slower running multiple level coder producing a plurality of encoded digits was utilized instead of a delta modulator, complex and expensive digital multipliers would be required instead of simple gates to provide the weighted accumulation that provides the high resolution PCM signal. Furthermore, in this arrangement even when a delta modulator is employed the statistics of the input analog signal must be known to select the most favorable coefficients of the multipliers to provide a PCM signal of high resolution by minimizing the cumulative effect of quantizing noise.
In another arrangement a number of code groups produced by a high speed multiple bit direct feedback coder are averaged. The averaging technique takes advantage of a self-cancelling characteristic imposed upon the quantizing noise by the configuration of the coder. This is due to the location of the integrator in the forward signal path of the quantizer instead of the feedback path of the subtractor. Hence, this is the basis for the name of direct feedback coder. The arrangement is therefore able to provide a signal of improved resolution without employing complex digital multipliers.
A primary object of this invention is to combine a number of multiple bit code groups produced by a direct feedback encoder to provide a PCM signal of higher resolution than was obtainable through prior art techniques.
A related object of the invention is to provide an arrangement that performs weighted accumulation of multiple bit code groups to produce a high resolution PCM signal without employing complex digital multipliers.